mtd: sharpslpart: Fix unsigned comparison to zero

The unsigned variable log_num is being assigned a return value
from the call to sharpsl_nand_get_logical_num that can return
-EINVAL.

Detected using Coccinelle:
./drivers/mtd/parsers/sharpslpart.c:207:6-13: WARNING: Unsigned expression compared with zero: log_num > 0

Fixes: 8a4580e4 ("mtd: sharpslpart: Add sharpslpart partition parser")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>